March 2018 - last edited March 2018
The more you interact, (quite a lot) they'll age up + (you'll gain xp) Sims are also experiencing this natural process in a different way. Sims will age up as you interact with other Sims, attend events and enhance their traits.
Hope this helps,
Sasha
note: babies, toddlers, and kids aren't playable. You'll have to wait until they get past the child stage in order to make them playable.
nb: If this solves your question, please mark the reply as having been the solution, so as to help others with a similar problem in the future.
I have no connection to EA